t: use hash-specific lookup tables to define test constants

t: use hash-specific lookup tables to define test constants

In the future, we'll allow developers to run the testsuite with a hash
algorithm of their choice.  To make this easier, compute the fixed
constants using test_oid. Move the constant initialization down below
the point where test-lib-functions.sh is loaded so the functions are
defined.

Note that we don't provide a value for the OID_REGEX value directly
because writing a large number of instances of "[0-9a-f]" in the
oid-info files is unwieldy and there isn't a way to compute it based on
those values. Instead, compute it based on ZERO_OID.

Signed-off-by: brian m. carlson <sandals@crustytoothpaste.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>